Classical eyeblink conditioning has become a powerful preparation for studying the neural bases of learning and memory in both animals and humans. A well-defined neural circuit involving the cerebellum and brainstem is sufficient to support simple associative learning, ie. delay conditioning. However, more complex forms of learning, such as trace conditioning, appear to involve forebrain systems involved in cognition, ie. the hippocampus. Because the cerebellum and hippocampus develop postnatally in most mammals, eyeblink conditioning may be an ideal model for studying the role of these structures in cognitive development. The main goal of this proposal is to use classical eyeblink conditioning to study early neurocognitive development in rats and humans. Comparative studies of the development of eyeblink conditioning will characterize the ontogeny of delay and trace eyeblink conditioning in both species. The studies are designed to test the hypothesis that trace conditioning will emerge later than delay conditioning, and that the ontogeny of trace conditioning is dependent on maturation of the hippocampal cholinergic system and its interactions with the cerebellum. Cross-sectional and longitudinal behavioral studies in rat pups at different ages will be used to determine the age at which pups are capable of forming the associations underlying trace and delay conditioning, versus the age at which these forms of learning come to be expressed. The proposed studies will also determine the effect of disrupting normal hippocampal development on the emergence of delay and trace conditioning. Finally, the ontogeny of delay and trace conditioning in health full-term human infants will be determined as a prelude to future studies of populations of infants at high risk for neurological damage of various etiologies.